Within the market for document of security and/or value, more particularly documents of identification (ID documents), the demand exists for continual improvement in the safety features used, and also for the development of new safety features in order to keep one step ahead of potential forgers at all times. The security features ought to be such that a forgery is technically difficult and in purely visual terms is extremely easy to identified.
The use of holograms in documents of security and/or value has for many years been a popular means of increasing the forgeproofness of these documents. The holograms here are typically adhered to the documents by means of hot embossing. To a large extent, the holograms used are embossed holograms which are provided on their reverse face with a heat-activatable adhesive. During the hot embossing operation, the adhesive is activated by the heat of the embossing die, and the hologram joins to the document and can then be parted from its backing film.
Embossed holograms themselves are molded into a thermoplastic in an embossing step from a precision mold (nickel shim). Surface holograms of this kind typically have an embossed thickness of less than one micrometer. The nature of these holograms, limits their light efficiency to around 30%. For this reason, these holograms are usually subsequently metallized or actually embossed into metallized films in order to increase their visibility. Furthermore, these holograms display a “rainbow-like” polychromatic color pattern. In order to increase the forgeproofness of the documents and the life of the holograms, holograms are also laminated into documents made of plastic. In that case the holograms are embossed onto the inner films, and then covered with transparent films in order to ensure their visibility. However, the forgeproofness of embossed holograms is not particularly high. Moreover, metallized embossed holograms with better visibility generally form significant weak points in documents made from plastic.
More recently, furthermore, a new class of holograms have been used in security cards, being distinguished by their high optical efficiency (diffraction efficiency >90%) and by their uniform and unambiguously definable coloredness. These holograms are called volume holograms, since their diffractive structure has been written into the entire layer thickness (usually 10-20 μm). Volume holograms display a light-diffraction behavior with a pronounced angular dependency. Accordingly, they are transparent at certain angles (off-Bragg) and clearly visible at others (on-Bragg). Such holograms are produced using special photopolymer formulations. WO-A 2011/054797 and WO 2011/067057, for example, describe such special photopolymer formulations for producing holographic media. As described in WO-A 2011/054797 and WO 2011/067057, photopolymer formulations of these kinds with a high difference in refractive index can be applied to a transparent substrate film.
In order to integrate holograms into documents of security and/or value, especially into bodies of ID cards, one possible operation that may be conceived is that of adhesive bonding with either a liquid adhesive formulation or dispersion or an adhesive film.
The possibility of integrating volume holograms into ID cards to produce a wavelength shift in the volume hologram is known from DE 10 20040 12787 A1. This method is utilized for subsequent individualization, but does not permit the hologram to be reproduced in its original colors, and is therefore unsuitable for the incorporation of “true color” holograms.
Since the method described also results in an imprecise reproduction of the hologram, holograms nowadays are generally bonded adhesively to the prefabricated card body, as described in specifications DE 10 2006 048464 A1 and WO 2008/043356 A1. However, since a hologram of this kind is on the outside and is not integrated completely into the fused card body, it is still amenable to attack by a forger. The adhesive bond becomes the potential weak point in the card assembly. Moreover, holograms that are merely adhered are easy to remove from the card body without destruction, and can be perished to endorse forged documents.
In the production of documents of security and/or value, more particularly identification documents in the form of cards (ID cards), the requirement exists for a further increase in forgeproofness by means of security features, such as holograms, for instance. Such use of holograms as security features, however, makes sense only if the holograms can be integrated into the card body and not merely—as described above—adhered to the card body.
Plastics-based documents of security and/or value, more particularly documents of identification, such as ID cards, for example, are nowadays preferably produced in the form of multilayer assemblies, without the use of layers of adhesive, by means of lamination at high temperatures and high pressure, in order to prevent subsequent separation of the layer construction for the purpose of switching of identification features. Incorporated into these multilayer assemblies before or during the laminating operation are the corresponding security features which, consequently, must be of a kind such that they withstand the parameters of the laminating operation without suffering destruction. Moreover, the security features must not introduce any weak points into the multilayer assembly that, again, would enable subsequent opening of the assembly without destruction.
In the case of integration into the card body as well, therefore, it is necessary to ensure that the hologram does not introduce a weak point into the multilayer assembly and that the card body cannot be separated and the hologram removed—without destruction of the hologram—and used to endorse a forged document.
Described in WO-A 2012/019588, for example, is the necessity of bonding a security element, such as a hologram, into the middle of a card assembly using reactive adhesives. This, however, requires an additional operating step, and precisely matched positioning during the subsequent laminating operation, and does not get around the adhesive bond as a potential weak point in the card assembly.